A discharge lamp for use as a light source for office automation equipment is keenly required to have a uniform luminance over the entire length thereof.
As a prior art relating to improvements of the luminance distribution of the discharge lamp, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 57-11465 (1982) discloses a system in which electrode filament coils are heated into white-incandescence to emit light, thereby compensating for the reduction in luminance which would tend to occur particularly at end portions of the lamp.
In the above-mentioned prior art, however, it is necessary to heat the electrode filaments to a color temperature of 2600.degree. to 3200.degree. K, and, therefore, it is necessary to provide another pair of filament coils for maintaining the life of the lamp, namely, for maintaining the discharge under the heated conditions. Accordingly, the prior art involves an increase in the number of component parts of electrode structure, with the result of a very complicated electrode structure, leading to a complicated manufacturing process and a rise in cost.